PSC motors have been widely utilized as motors for automatic washing machines. In order to provide multi-speed operation for various washing circles without a complex and expensive variable frequency speed regulator, certain type of washing machines comprise a number of completely separate stator windings for different poles which operate at each speed. For an example, in a 3-speed PSC motor instance, a stator is provided with an eight-pole winding for operation at low speed, a six-pole winding for operation at medium speed, and a four-pole winding for operation at high speed. Usually, to accommodate a large number of windings in the limited slot size, the windings are typically made of copper, which is relatively costly.
Common winding configurations have been proposed in the prior art, wherein part of the windings are shared for operation at different number of stator poles by changing the connection, thereby achieving different motor speeds (Referring to Chinese Public Specification of an Invention Patent Application CN1,479,434 A). By comparison with conventional configurations, common winding configurations have substantial advantages such as reduced motor size, improved material efficiency and decreased product costs, etc.
However, the feasibility of common winding techniques largely depends on whether the switch among various operation modes or reconnection of the windings is simple or not. FIG. 1 is a schematic diagram of a prior art PSC motor with 4/6 pole common windings. As can be seen from FIG. 1, five single-pole double-throw switches and one single-pole single-throw switch are necessary to switch between 4-pole and 6-pole configurations. Further, to provide improved performance by using capacitors with variable capacity at 4-pole and 6-pole operation modes, an additional single-pole single-throw switch may be provided.
Unfortunately, the prior art relays with six or more sets single-pole double-throw contacts are complex, large, and costly. Therefore, there exists room for improving the common winding configuration.